Several types of mountings and means for centering the arc discharge tube within an outer envelope in an arc discharge lamp are well known in the art. Examples are illustrated in U.S. Pat. Nos. 3,662,203 (Kuhl et al), 3,932,781 (Peeters et al) and 4,401,913 (Koza et al). In Kuhl et al and Peeters et al, means for centering the arc discharge tube also serve as current supply members for the electrodes of the arc discharge tube. In both embodiments, the current supply members or expansion lead-ins are made of molybdenum. In Kuhl et al, the lead-in members are "S" shaped; and in Peeters et al, the lead-in members are spiral shaped. In both inventions, the arc tube centering means is connected to the arc tube and to the outer lead assemblies located at each end of the lamp's outer envelope.
The aforementioned patents do not address the difficulties encountered during lamp sealing in attempting to center the arc tube within the outer envelope while maintaining the integrity of the seal in the outer lead assemblies. In addition, the problem of premature failure of an arc tube centering means contained within the lamp due to fatigue, caused by constant compensation for the thermal expansion of an arc tube with respect to the outer envelope, has so far remained unsolved.
Providing an effective and lasting electrical connection between the arc tube centering means and the arc discharge tube has also been a difficult problem. In order to center the arc tube, some tension must be applied to the outer lead assemblies. As heat is applied during lamp sealing, the coiled expansion leads expand and tend to move the molybdenum sealing ribbon, located within the outer lead assembly, out of alignment and make a poor seal. In addition, the molybdenum expansion leads may not make very good welds with the niobium feed-through tubes of the arc tube. Since niobium is soft and has no spring temper, it was believed that effective expansion leads could not be made from such a material. Finally, during heat sealing the coiled expansion leads expand and tend to move the arc tube out of its center position.
Arc tube centering means for an arc discharge lamp which improves the integrity of the seal located at the outer lead assembly and which improves the integrity of the electrical connection between the centering means and the arc tube would constitute a significant advancement in the art. Such a lamp would be an even further advancement if it could be easily manufactured, would prolong the life of the arc discharge lamp, and prevent premature failure of the arc tube centering means due to fatigue.